Firstly, next time you post, please read the rules. You have posted in the wrong forum as this is most defiantly XMBC related and therefore belongs in the XMBC forum (where there is a nice post template to provide some basic information I need when answering such questions!)

Secondly, isn't that the default behavior in windows for button 4 and 5 (or is that the reverse of normal?)
Thirdly, with nothing to go on, I have no idea what to suggest.

Do the XMBC button setting dropdowns highlight in orange when you press them? If not then XMBC isn't seeing the button presses and that means either your mouse isn't sending them (you don't mention the make of your mouse for example, Logitech mice buttons can be reconfigured externally using SetPoint (or whatever its called these days).

If they do highlight in orange, then XMBC is seeing them, so then the question will be why isnt the rempa working. If thats the case, maybe you could turn on debug logging and then press the buttons a few times and send me a copy of the log - perhaps that will help me understand whats going on!